How ISPs detect that i am using Hotspot or Tethering?

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1

So, whenever i see the plan lists from ISPs or Mobile Carriers, it always limits data usage amount for Hotspot and Tethering.

I have no idea how they check this, and i have tried test by myself.

I've enabled Hotspot at my phone and connected with my laptop and tablet.

when used ipconfig at my laptop, i can see my address is 192.168.X.X, and i also can see similar at my tablet.

However, when i type like "what's my ip address?" at google,
phone, laptop, and tablet's ip addresses are all different.

I assumed it will show same ip address like i'm using a router, but it's not.

I still have no idea how this works.

One more,
If i use a LTE or 5G router, like egg, which is operated with OpenWrt, will same thing happen like above?

2

They are looking for two things:

  1. TTL of the outgoing IP packets.
  2. Requests for things (like time.microsoft.com and certain antivirus updates) that don't make sense on mobile.
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3

Mobile companies certainly can, but not necessarily so, do what @patrakov says.
But most companies do add OS extensions that inform them when hotspot/tethering is turned on.

4

I think it is country-dependent. In countries where carrier-locked phones are the norm, perhaps @bluewavenet is right. But for Russia, the predominant technique is based on the TTL.

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5

If you use a supported openwrt router with an usb or internal modem (cell phone via usb should work just fine BUT NOT via WIFI as it counts as tethering) you can easily bypass any ttl checks that your isp employs.
There are as well methods for rooted phone but that's outside the scope of this forum.